dronweA U.S. drone killed four suspected al Qaeda militants on Monday in a strike in central Yemen, tribal sources said, in the third such attack in three days.

The United States never comments on strikes by its pilotless aircraft, which it has used to track down militants in Yemen for years. The Yemeni government tolerates such strikes but usually does not comment on the U.S. role in specific incidents.

Washington has scaled up action against al Qaeda in the strategically important state, where the group exploited widespread anti-government unrest in 2011 to seize swathes of territory in the south.

It was subsequently driven out by a military offensive in June last year.

Yemen’s defense ministry said in a statement the air strike targeted a vehicle being used by militants in al-Ateef in the central province of Maarib, killing four fighters. It did not say who was responsible.

Tribal sources said a pilotless plane had attacked a car on a main road linking the capital Sanaa with Maarib.

“The strike burnt the car which was carrying four suspected al Qaeda (militants) and killed those in it,” a source told Reuters.

Six suspected insurgents were killed in two other drone strikes in Maarib on Saturday and Sunday. A further 10 suspected al Qaeda fighters died in an explosion in a house in southern Yemen on Sunday.

Shoring up stability and security in Yemen is a priority for the United States and its Gulf Arab allies because of its location next to the world’s top oil exporter, Saudi Arabia, and to shipping lanes, and because it is home to one of the most active wings of al Qaeda.

But local discontent at the drone strikes is growing.

On Sunday armed tribesman, angry at what they said was a drone attack on an area inhabited by civilians, blocked the main road linking Maarib with Sanaa.

Earlier this month, dozens of armed tribesmen also took to the streets in southern Yemen to protest against drone strikes that they said had killed innocent civilians and fuelled anger against the United States.

Al Qaeda in the Arabian Peninsula (AQAP) is considered by Western governments to be one of the most active and dangerous wing of the global network founded by Osama bin Laden, and has attempted a number of attacks against U.S. targets.